1. Field of the Invention
The present invention relates to a steering torque detector for the electric power steering gear of a vehicle such as a motor vehicle, and particularly relates to a steering torque detector improved so that the ends of sliders, which are kept in contact with the neutral points of the resistor layers of a potentiometer having two circuits, are located on the same radius of the detector.
2. Prior Art
In a conventional steering torque detector disclosed in the Japanese Utility Model Application No. 91811/87, the input and output steering shafts of a steering gear are coupled to each other by a torsion bar. A printed circuit board provided with elements of a potentiometer is attached to the input steering shaft. Sliders are attached to a mounting ring attached to the output steering shaft. The tips of the sliders, which project out in parallel with the axis of the detector, are kept in contact with resistor layers which are some of the elements of the potentiometer. When the torsion bar is twisted by moving the steering wheel of the steering gear, the sliders in contact with the resistor layers are displaced in the circumferential direction of the detector so that detection signals proportional to the quantity of the displacement are sent out from the potentiometer.
FIG. 1 shows the printed circuit board 14 for forming the main and auxiliary circuits of the potentiometer. The board 14 is shaped as a disk and secured to the flange of a support ring fitted with slip rings. The potentiometer elements 15, which are for forming the main and the auxiliary circuits, are provided on one side of the printed circuit board 14 and located on mutually concentric circles. Some of the potentiometer elements 15 are the resistor layer 15a and electrode 15d of the main circuit. The others of the potentiometer elements 15 are the resistor layer 26a and electrode 26d of the auxiliary circuit. The resistor layers 15a and 26a are located at a circumferential distance from each other on the same circle. The electrodes 15d and 26d are located at radial distances from the resistor layers 15a and 26a and extend throughout the same ranges of central angle of the printed circuit board 14 as the resistor layers. A positive power input terminal T1 and a negative power input terminal T2 for the potentiometer are connected to a detecting power supply through lines 28. Signal output terminals T3 and T4 for the potentiometer are connected to the main and the auxiliary circuits through other lines 28. The printed circuit board 14 has a notch 14a through connection lines extending from the slip rings are laid. The board 14 also has a notch 14b for positioning the board relative to the flange of the support ring.
FIG. 2 shows the main slider 16 and auxiliary slider 27 of the conventional steering torque detector. The sliders 16 and 27 are also for forming the main and auxiliary circuits of the potentiometer. The mounting ring 17 made of an electric insulator is fitted around the output steering shaft and secured by screws not shown in the drawings. The main slider 16 and the auxiliary slider 27 are attached to one side of the mounting ring 17 so that the tips of the projecting portions of the main slider are located in contact with the resistor layer 15a and the electrode 15d which are located away from each other in the radial direction of the printed circuit board 14, and the tips of the projecting portions of the auxiliary slider are located in contact with the resistor layer 26a and the electrode 26d which are located away from each other in the radial direction of the board. The circumferential length of the area of the contact between each of the tips of the projecting portions of the main and the auxiliary sliders 16 and 27 and each of the resistor layers 15a and 26a and the electrodes 15d and 26d is very small.
FIG. 3 shows the main and auxiliary circuits of the potentiometer, which are composed of the potentiometer elements 15 and the sliders 16 and 27. The detection signal of the main circuit is sent out from the signal output terminal T3, while that of the auxiliary circuit is sent out from the other signal output terminal T4.
When the steering wheel is not moved, the sliders 16 and 27 are in contact with the neutral points of the resistor layers 15a and 26a, respectively. When the steering wheel is moved, the torsion bar is twisted depending on the torque of the wheel, so that the sliders 16 and 27 are displaced by the same angle as each other from the neutral points of the resistor layers 15a and 26a as the sliders remain in contact therewith. At that time, the detection signal of the main circuit is compared with that of the auxiliary circuit. If the detection signals are equal to each other in level, the main circuit is judged to be normal, so that a steering electric motor is rotated by the detection signal of the main circuit. If the detection signals are not equal to each other in level, at least one of the resistor layers 15a and 26a is judged to be abnormal, so that a warning is displayed or sounded. The reliability of the steering torque detector is thus enhanced.
FIGS. 4 and 5 show the slider 16 of the conventional steering torque detector is attached to a mounting ring 17, as shown in FIGS&gt; 4 and 5. The mounting ring 17 is made of a synthetic resin and has a slender hole 17b provided in the body 17a of the ring and slenderly extending in the circumferential direction thereof. After the mounting ring 17 is fitted on the output steering shaft 2, the position of the mounting ring is adjusted relative to the shaft in the circumferential direction thereof so that the end of the slider 16 is placed on the neutral point of the resistor layer. The mounting ring 17 is then secured to the output steering shaft 2 by a screw 18.
FIG. 6 shows a longitudinally sectional view of a conventional steering torque detector disclosed in the above mentioned Japanese Utility Model Application No. 91811/87. In the detector, the input and output steering shafts 1 and 2 of a steering gear are coupled to each other by a torsion bar 3. A pin 4 is secured to the input steering shaft 1 and the torsion bar 3. A housing 6 is supported on a fixed section not shown in the drawing and supports the input steering shaft 1 with a bearing 8. Another housing 7 is coupled to the former housing 6 by screws 10 and supports the output steering shaft 2 with a bearing 9. A support ring 11 for a plurality of slip rings 12 is molded from a synthetic resin and secured to the input steering shaft 1 and has a flange 11a. The slip rings 12 are embedded in the support ring 11 and connected to electric wires 13. A printed circuit board 14 shaped as a disk is secured to the flange 11a of the support ring 11 and provided with elements 15 of a potentiometer. A resistor layer extending in the circumferential direction of the printed circuit board 14 and a plurality of electrodes are the potentiometer elements 15. A mounting ring 17 made of an electric insulator is fitted around the output steering shaft 2 and secured thereto by a screw 18. A slider 16 is attached to the mounting ring 17 and extends from the resistor layer of the potentiometer to the electrode thereof so that the slider is in contact with the resistor layer and the electrode. A brush unit 19 includes a bush holder 20 made of an electric insulator and attached to the housing 6, brushes 21 made of thin elastic wires of high electric conductivity and having a spring-like property, and terminals 22 embedded in the brush holder and connected to lead wires 24 through capacitors 23. The brushes 21 extend in contact with the slip rings 12 at the tips of the brushes, in the tangential directions of the slip rings, and are secured to the terminals 22 at the butts of the brushes. A cover 25 is provided around the brush unit 19.
FIG. 7 shows the slip ring 12 and the brush unit 19. Each of the brushes 21 of the brush unit 19 is made of the thin elastic wires divergently extending in two mutually different directions from the butts of the wires toward the tips thereof and elastically deformed in contact with the peripheral surface of the corresponding slip ring 12. Before the thin wires of the brush 21 are elastically deformed into contact with the peripheral surface of the slip ring 12, the wires extend as shown by one-dot chain lines in FIG. 7.
However, the conventional detector as described above has the following problems:
First, since the main resistor layer 15a and the auxiliary resistor layer 26a are located at the circumferential distance from each other and the main slider 16 and the auxiliary slider 27 are also located at a circumferential distance from each other, it is not easy to place the sliders in contact with the neutral points of the resistor layers and requires highly accurate processing and assembly. Besides, if the printed circuit board 14 and the mounting ring 17 become eccentric to each other, the tips of the sliders 16 and 27 deviate from the neutral points of the resistor layers 15a and 26a to lower the accuracy of the output characteristic of the steering torque detector.
Also, the main and auxiliary circuits of the potentiometer of the conventional steering torque detector need to be equal to each other in signal characteristic in the normal state of the detector assembled. For that reason, it is necessary to make the processing and assembly of the components of the detector. This results in lowering the productivity for the detector.
Further, in the conventional detector, it is troublesome and time-consuming to finely adjust the position of the mounting ring 17 relative to the output steering shaft 2 in the circumferential direction thereof. Besides, the screw 18 for securing the mounting ring 17 to the output steering shaft 2 is likely to loosen due to the long period use of the detector or the secular shrinkage of the mounting ring so as to let the ring be displace in the circumferential direction of the shaft to shift the position of the slider 16 from the neutal point of the resistor layer. For that reason, the detection signal is likely to become an improper one.
Still further, in the conventional detector, the inner end of the boss of the mounting ring of the steering torque detector and the surface of the printed circuit board thereof, which is kept in pressure contact with the outer end of the boss under the pressure of the spring which is applied to the ring and the board, become worn because of slipping on each other as the input and the output steering shafts are turned relative to each other along with the twisting of the torsion bar at the time of the movement of the steering wheel of the steering gear. Therefore, the life of the detector is shortened. This is a problem.
Still further, in the conventional detector, the distance between the tips of the thin elastic wires of each brush 21 before the brush unit 19 is attached to the housing 6 so as to elasticaly deform the wires into contact with the peripheral surface of the corresponding slip ring 12 is smaller than that after the brush unit is attached to the housing so as to elastically deform the wires into contact with the peripheral surface of the slip ring. For that reason, it is likely that before the brush holder 20 is put into the insertion hole of the housing 6, the tips of the thin elastic wires of each brush 21 do not go into prescribed positions of the corresponding slip ring 12, but go into contact with a wrong portion and bend or go onto a wrong slip ring to cause a short circuit or an improper signal.